This invention relates to apparatus which may be used to assess visual function and ocular motility, and to treat disorders thereof. The invention also relates to methods of treatment of such disorders that involve the use of that apparatus.
In particular, the invention relates to ocular display apparatus which presents different, but visually related, images to each eye. The invention also relates to methods of assessment of ocular disorders and to their treatment.
There are a number of common eye conditions in which a person's eyes fail to work correctly in combination with each other. These conditions can result in blurred or double vision, or the absence of true stereovision.
Such conditions are often the result of physical problems, ie strabismus, commonly known as a squint, which can result in amblyopia, commonly known as “lazy eye”.
This invention has particular relevance to the assessment and treatment of amblyopia. However, the invention may also be applicable to the study and treatment of many other eye conditions, and vision/coordination related brain problems (particularly for survivors of stroke), or neurological pathway developmental problems.
Amblyopia is a condition of the visual system in which one eye fails to develop a normal level of visual acuity during the developmental period for vision. Amblyopia can occur in subjects who are strabismic (have a squint) or have anisometropia (such that both eyes have different refractive errors, leaving one eye defocused), or a combination of both (mixed amblyopia). Amblyopia can also arise from stimulus deprivation (for example, as a result of having a cataract). The poor vision resulting from amblyopia does not always resolve even when the underlying condition has been treated.
Amblyopia is a common condition of childhood, affecting perhaps as many as 2-3 percent of the population, and can carry over into adult life if left untreated. Whilst most people can manage with a lazy eye, they may well have reduced or no binocular function and this may compromise their ability to perform certain complex tasks, such as flying an aeroplane or driving a train. Furthermore, persons with one amblyopic eye who suffer injury to their “normal” eye may be effectively blind.
A person's visual field can be divided into peripheral and central vision. In amblyopia, the peripheral vision is normal but there is a defect in the central vision with reduced sensitivity to detection of stimuli with high spatial frequency, which is reflected in reduced visual acuity, and the presence of a central area of reduced visual sensitivity (scotoma).
Whilst many treatments for amblyopia have been attempted, the most common remains the use of a patch, which has been advocated for decades. Indeed, use of a patch for this purpose was documented as early as the eighteenth century. In such treatments, the non-amblyopic eye is covered with a patch for prescribed periods of time daily (eg several hours each day) over a protracted period, perhaps for several months or even years. The patching of the non-amblyopic eye forces the wearer to use the amblyopic eye, and hence that eye is stimulated, leading to an improvement in function. A major drawback with the use of the patch, however, is poor subject compliance where the subject does not wear the patch for the prescribed time periods.
Most subjects treated in this way are children. Many children do not wear the patch as directed, for instance because their vision is much poorer when using only the lazy eye, or because they are teased when wearing the patch and so remove it. Non-compliance is a major cause of failure of the patching technique in treating children with amblyopia.
An alternative treatment is penalisation, in which the vision in the normal eye is blurred by the administration of atropine drops. This treatment was introduced in the 19th century and is less commonly used than patching, but the reported results are similar.
It is thought that children with amblyopia are best treated before the age of eight years. Patients with amblyopia who are over the age of 12 years do not respond to patching or penalisation.
Dichoptic stimulation, the presentation of different images to each eye, has been shown to be effective in the treatment of amblyopia. See WO03/092482, which describes an ocular display apparatus having image presentation means adapted to display a first image to one eye only of a subject, and a second, different image to the other eye only. The first and second images are presented to the subject so that they perceive a composite image, at least one of the first and second images including a moving object. The apparatus is useful in the treatment of amblyopia and other ocular disorders. However, a major limitation of this apparatus is an inability to compensate for the presence of strabismus.